There is presently a trend in manufacturing towards achieving increased product velocity in order to reduce work-in-process, and hence, manufacturing costs. As product velocities increase, visual product inspection by an operator to detect defects, after one or more fabrication operations are performed, becomes increasingly difficult. The need to achieve accurate product inspection rapidly, on a repeated basis, has led to the development of machine vision inspection systems. Such systems typically incorporate an image-acquisition device, in the form of a television camera or the like, and a vision processor, usually a microprocessor, programmed to process the output signal of the camera to obtain the image of the product being inspected. The image of the product is processed by the microprocessor to detect whether there are any visual defects which might render the product defective.
In most manufacturing facilities, the ambient lighting is often of insufficient intensity and is not structured to enable the camera of the machine vision system to obtain a high contrast image of the product undergoing inspection. Therefore, most vision inspection systems incorporate one or more separate illuminators for illuminating the product undergoing inspection. In many instances, a fiber optic illuminator is employed for this purpose. The typical fiber optic illuminator comprises an electric light bulb, often fabricated together with a mirror or integral reflector, which serves to reflect the light radiated by the bulb into a length of lightguide fiber which carries light to the product. In some instances, associated optics may be provided to focus the light reflected by the mirror into the fiber.
With present day fiber optic illuminators, the light bulb is energized from a voltage supply powered from an AC line. Fluctuations in the AC line voltage, which are common in most industrial environments, will cause variations in the intensity of the light radiated by the light bulb. Even small variations in the intensity of the light from the light bulb of the fiber optic illuminator can adversely affect the image of the product captured by the camera of the vision inspection system, and hence the accuracy of inspection. Further, as the electric light bulb of the illuminator ages, the intensity of the light radiated by the light bulb also changes, which can also affect the accuracy of product inspection.
Therefore, there is a need for a technique for illuminating a product undergoing inspection with light of substantially constant intensity, notwithstanding variations in line voltage and the effects of light bulb aging.